1. Field of the Invention
The present invention relates to an image scanner, and more particularly, to a such a scanner in which dither plates are utilized to steer an image onto an image sensor.
2. State of the Prior Art
In image scanners which utilize solid-state image sensors, glass plates, known as dither plates, are used to move an image relative to the image sensors in order to fill in areas between individual sensors and thereby gain additional pixels of information. The plates are moved through a small angle to scan the image, and they must be precisely controlled both as to position and velocity.
Scanner dither plates have been driven by stepper motors. For example, in U.S. Pat. No. 4,638,371, an image scanner is disclosed in which two dither plates are moved in orthogonal directions by a pair of stepper motors. In driving a dither plate with a stepper motor, it is necessary to use a microstepping controller to achieve the small step size required in driving the plates. Such a controller adjusts the currents applied to the windings of the motor so as to position the rotor in increments which are smaller than the rated step size. A problem with this arrangement is that stepper motors have an undesirable tendency to overshoot, or even oscillate, at the end of a step. When used in a microstepping application, the amount of overshoot can be excessive. Attempts have been made to overcome this problem by the use of dampers or by customizing the command signal; however, such solutions have not been completely satisfactory.